1. Field of the Invention
The present invention generally relates to a method of continuous casting steel. Specifically, the present invention creates an important improvement in continuous casting in which magnetic poles are attached to the outer surface of a pair of opposing side walls of the mold and a straight immersion nozzle is employed, which art is adopted for continuous casting of a low C-Al killed steel. This is done with a view to assuring that, even when high-speed continuous casting is performed by, for example, increasing throughput per unit period of time, defects of products (such as sliver and blister) can be prevented from often occurring due to an increase in the amount of accumulatively trapped inclusions and/or an increase in the amount of included powders or bubbles.
2. Description of the Related Art
In general, measures for preventing such defects of products include the following:
(1) Purifying the molten steel to a higher degree by ladle refining PA1 (2) Employing a tundish of a greater capacity so as to prevent contamination by ladle slag and tundish powder, and PA1 (3) Improving the configuration of the immersion nozzle so as to prevent entrapping of various inclusions and powders into the mold
However, these conventional measures can improve the purity of the molten steel used in a production process only to a limited extent when the process is adapted to meet various requirements such as the required levels of product quality and production quantity. Thus, these measures cannot be regarded as perfect measures.
In addition, once various inclusions and entrapped powders are brought into the mold, they cannot completely surface when the throughput per unit period of time is increased beyond a certain limit. In this case, therefore, these substances tend to be trapped in the steel.
A method has conventionally been proposed as a means of overcoming these problems. Electromagnets are disposed on the mold of a continuous slab casting machine, and a traveling magnetic field is applied to the molten steel in the mold in such a manner that the flow of the molten steel is controlled by the Lorentz force generated by the interaction of the current induced in the molten steel and the magnetic field. This makes it possible to prevent the flow of discharged molten steel from deeply penetrating the molten steel pool, thereby preventing the entrapping of mold powder and promoting surfacing of the various inclusions.
This conventional method is put to practice as indicated by the following examples:
(i) When a two-hole nozzle is used as the immersion nozzle, a traveling magnetic field is applied to a region corresponding to the full width of the wide face walls of the mold, and the magnetic field is caused to travel in the widthwise direction of the wide face walls of the mold (see page 356 of "Proceedings of the Sixth International Iron and Steel Congress (IISC), 1990")
(ii) When a two-hole nozzle is used as the immersion nozzle, a traveling magnetic field is applied to a region corresponding to part of the full width of the wide face walls of the mold, and the magnetic field is caused to travel in a vertical direction with respect to the direction of casting (see page 309 of "Proceedings of the Sixth IISC, 1990")
The first method (i) employs, as shown in FIG. 5, an immersion nozzle 2 comprising a two-hole nozzle having an ejection hole 2a on each side. Magnetic poles 5 for generating a traveling magnetic field are disposed in an area corresponding to the full width of the wide face walls (not shown) of the mold which are held between narrow face walls 1a of the same and including the position of the ejection holes 2a of the nozzle 2. A magnetic field generated by the magnetic poles 5 is reciprocated in a widthwise direction relative to the steel piece being cast, that is, in a horizontal direction, thereby accelerating or decelerating the flow of the molten steel ejected from the ejection holes 2a of the nozzle 2, so as to prevent inclusions 14 or bubbles 15 from entrapping with the molten steel 16 in the mold or to effect the compensation of the molten steel heat regarding the meniscus 7.
According to the FIG. 5 method, when the flow of discharged molten steel is decelerated by the traveling magnetic field, the magnetic field acts as a reflecting plate with respect to the molten steel flow. As a result, the molten steel flow is divided into an upwardly flowing stream 12 and a downwardly flowing stream 13. The upwardly flowing stream 12 causes mold powder to be entrapped at the meniscus 7, while the downwardly flowing stream 13 causes inclusions 14 and bubbles 15 to penetrate into the mold. There is a risk that these substances will be trapped by or in the solidified shell 6.
Conversely, when the flow of discharged molten steel is accelerated by the traveling magnetic field, although heat compensation at the meniscus 7 can be ensured, an increased amount of reversing current occurs on the narrow face walls 1a. This results in entrapping of mold powder and the penetration of inclusions and bubbles being promoted.
The second method (ii) also employs, as shown in FIG. 6, an immersion nozzle 2 comprising a two-hole nozzle having an ejection hole 2a on each side. In this case two magnetic poles 5 are provided for generating a traveling magnetic field. They are disposed in an area corresponding to a part of the full width of wide face walls (not shown) and comprise sections on either widthwise side of the position of the nozzle 2. The magnetic field generated by the two magnetic poles 5 is traveled in a downward direction with respect to the direction of casting, thereby decelerating that part of the flow of the molten steel ejected from ejection holes 2a of the nozzle 2 and heading toward narrow face walls 1a of the mold to collide therewith.
According to the FIG. 6 method, since the magnetic field is not applied in the full width of the wide face walls of the mold, the regions which are not acted upon by the magnetic field involve an upward stream 12 or a downward stream 13 of the molten steel, thereby failing to satisfactorily prevent the entrapping of mold powder at the meniscus 7 or the penetration of inclusions 14 and bubbles 15 into the molten steel in the mold.
The use of a two-hole nozzle as the immersion nozzle in the conventional methods (i) and (ii), as shown in FIGS. 5 and 6, respectively, has the following disadvantages: (a) one-sided flow may occur in the molten steel in the mold due to nozzle clogging; and (b) since argon (Ar) gas is introduced through an Ar gas supply port (as indicated by reference numeral 4 in FIG. 5), there is a risk of blisters on the cast steel and other surface defects occurring.
Inclusions and bubbles may be penetrated deeper into the molten steel in the mold when there is a one-sided flow in the mold due to an imbalance, caused by nozzle clogging, between the respective ejection areas of the two ejection holes of the immersion nozzle, or there is a change in casting speed, or the width of slab cast is changed.
The immersion nozzle for forming a flow passage between the tundish 3 containing the molten steel and the continuous casting mold 1, as shown in FIG. 5, is usually formed of a refractory material, in the continuous casting of steel. With such an immersion nozzle, alumina tends to adhere to the inner surface of the nozzle particularly during the continuous casting of an Al killed steel. As a result, the flow passage of the molten steel becomes increasingly narrower as time passes from the start of a casting operation, thereby making it impossible to attain a desired flow of molten steel.
Severe adhesion of alumina occurs at a location where the flow of the molten steel deflects and, accordingly, tends to stagnate. When a two-hole nozzle is used, such a location is the vicinity of the ejection holes of the nozzle.
In order to cope with the problem of the clogging of a two-hole immersion nozzle, the conventional practice has usually included, as previously described, the step of bubbling an inert gas such as argon, into the molten steel supplied through the nozzle. However, when the feed rate of the inert gas is great, some of the inert gas may not surface to the molten steel surface, and part may be trapped by the solidified shell 6 (such as that shown in FIG. 5) in the mold, thereby involving the risk of a defect of the final product. Further, nozzle clogging cannot be sufficiently prevented by merely supplying an inert gas into the nozzle, and it is necessary to replace the nozzle frequently. When the immersion nozzle is of the two-hole type, such as the immersion nozzle 2 (shown in FIGS. 5 and 6) having two ejection holes 2a at symmetrical positions on either side of the forward end of the nozzle, the immersion nozzle is vulnerable to asymmetrical clogging of the ejection holes, thereby involving problems such as reduction in the product quality.
One form of effort to overcome the above problems involves the use of a nozzle containing CaO capable of reacting with alumina to form a compound having a low melting point. However, the use of such a nozzle has not been able to achieve effective results. Among other efforts, Japanese Patent Laid-Open No. 60-92064 discloses a method of pouring a molten metal adapted to restrain nozzle clogging. In this method, a DC magnetic field is applied to the flow of a molten steel within the nozzle so as to transform the molten steel flow into a laminar flow. With this method, however, since the flow of the molten steel descends deep into the crater of the molten metal in the mold, there is a risk of the accompanying inclusions failing to surface and becoming trapped by a solidified shell.
On the other hand, it has not been possible to use a straight immersion nozzle having an open end provided at the forward end of the nozzle body to constitute a discharge hole for the molten steel. This is because the flow passage within the nozzle has no bend, and the flow of discharged molten steel heads vertically downwardly toward the exit of the mold. As a result the inclusions in the molten steel, gas bubbles, etc. penetrate deep into the crater, involving the risk of an internal defect of the sheet steel product. Further, since the solidified shell is washed by the high-temperature molten steel flow heading vertically downwardly, the washed portion of the shell is hindered from solidifying, involving the risk of breakouts being generated, which makes casting impossible.